The present invention relates to a method for fabricating a semiconductor device containing silicide of a metal having positive oxide generation energy larger than the oxide generation energy of silicon.
In a transistor for a silicon LSI, a salicidation process for forming refractory metal silicide over a polysilicon gate and source/drain regions has been suggested in order to reduce the resistance of the polysilicon gate and that of a diffusion layer for the source/drain regions. Among other processes, a cobalt (Co) salicidation process for forming cobalt disilicide (CoSi.sub.2) is expected to be effectively applicable as technique that makes it easy to reduce the resistance of a polysilicon gate having a small line width and the resistance of a diffusion layer.
Hereinafter, a conventional Co salicidation process will be described with reference to FIGS. 6A through 6E (see, for example, A. C. Berti et al., Proceedings of IEEE VLSI Multilevel International Conference 1992, p. 267).
First, a transistor structure shown in FIG. 6A is formed. The transistor structure includes: element isolation regions (LOCOS) 3 formed on the surface of a silicon substrate 1; source/drain regions 2 formed in the substrate 1; a gate oxide film 4 covering a channel region; a gate electrode formed on the gate oxide film 4; and insulating sidewall spacers 6 formed on the sides of the gate electrode 5.
Next, after native oxide films on the gate electrode 5 and on the source/drain regions 2 are removed, a cobalt (Co) film 7 and a titanium nitride (TiN) film 8 are sequentially deposited over the entire surface of the substrate 1 by sputtering technique as shown in FIG. 6B.
Subsequently, as shown in FIG. 6C, the substrate 1 is heated in an inert gas (e.g., nitrogen) ambient in accordance with rapid thermal annealing (RTA) technique, thereby conducting a heat treatment at 500.degree. C. for about 60 seconds. The temperature rise rate is set at 10.degree. C./sec. or more. As a result of the heat treatment, reaction is produced between the silicon substrate 1 and the cobalt film 7 and between the gate electrode 5 and the cobalt film 7 whereby cobalt silicide (CoSi) 9 is formed.
The inert gas such as nitrogen has been introduced into a lamp annealer so as to suppress the oxygen concentration in the ambient to a low level. However, 5 ppm or more of oxygen, corresponding to 5.times.10.sup.-6 pressure of oxygen under atmospheric pressure, unintentionally remains in the annealer. Ti, generally used for salicidation, is less likely to be adversely affected by oxidation, whereas Co is very likely to be oxidized. Thus, Co is oxidized with oxygen at such a low level. In view of this fact, before the heat treatment for salicidation is performed, the cobalt film 7 needs to be covered with the TiN film 8 functioning as an antioxidant film.
Then, as shown in FIG. 6D, the unreacted portions of the Co film 7 and the TiN film 8 are removed by wet etching using a mixture solution of H.sub.2 SO.sub.4 --H.sub.2 O.sub.2 at 120.degree. C., for example.
Finally, as shown in FIG. 6E, a heat treatment is conducted at 750.degree. C. for about 30 seconds through rapid thermal annealing at 10.degree. C./sec. or more within an inert gas (e.g., nitrogen) ambient. During this process step, the CoSi film 9 further reacts with silicon in the silicon substrate 1 and in the gate electrode 5, thereby forming a CoSi.sub.2 (cobalt disilicide) film 10. Since the specific resistance of the CoSi.sub.2 film 10 is as low as about 16 .mu..noteq.cm, resistance reduction is realized in the source/drain regions 2 and the gate electrode 5.
In the Co salicidation process, a CoSi film is firstly formed by conducting the initial rapid thermal annealing at a relatively low temperature of 400.degree. C. to 600.degree. C. The reason is as follows. If a heat treatment is conducted at about 700.degree. C. to try to form a CoSi.sub.2 film from the beginning, then the resultant CoSi.sub.2 possibly laterally grows on the element isolation regions 3 and the insulating sidewall spacers 6 so as to shortcircuit the source/drain regions 2 and the gate electrode 5.
Recently, it was reported that a CoSi.sub.2 film can be formed without causing such an unwanted lateral growth even when such rapid thermal annealing is conducted from the beginning at as high a temperature as 700.degree. C. or more by depositing a Ti film having a thickness of about 2 nm under a cobalt film (see, for example, S. Ogawa et al., Extended Abstracts of International Conference on Solid State Devices and Materials, p. 195, 1993). However, in such a case, it is still considered necessary to cover the surface of the cobalt film with a TiN film or the like during the rapid thermal annealing, in order to suppress the oxidation of the surface of the cobalt film.
The technology illustrated in FIGS. 6A through 6E requires a process step of depositing a TiN film on a cobalt film, which in turn requires to separately provide a Ti target for a sputtering apparatus. However, when the TiN film is sputtered, particles are likely to be generated within the sputtering apparatus. If the number of particles increases within the sputtering apparatus, then the production yield of a semiconductor device ultimately tends to decrease.
Moreover, according to another suggested method (see, for example, K. Inoue et al., IEDEM, p. 445, 1995), a cobalt film is firstly deposited within a sputtering apparatus including a rapid thermal annealing chamber and a semiconductor substrate is moved into the rapid thermal annealing chamber and subjected to a heat treatment without breaking vacuum. In accordance with this method, a CoSi.sub.2 film can be formed without depositing a TiN film. However, since this method conducts a heat treatment by moving a semiconductor substrate within vacuum, the processing performance is not high to such an extent as to be appropriately applied for mass production.
In view of the above-mentioned conventional problems, the present invention was made in order to provide a method for fabricating a semiconductor device allowing for a salicidation without covering a metal film with an antioxidant film.